The invention relates to a method of compensating the proximity effect in the imaging of patterns in electron beam projection systems.
In the production of integrated circuits, the reduction of manufacturing costs and increase of operating speed demand structures of continuously decreasing size. Conductive lines with a width of 1 micrometer and less can no longer be resolved by means of present-day photolithographic methods.
Although in principle lithographic processes with electron beams permit a much higher resolution in the transfer of patterns, there actually exist considerable difficulties due to scattering effects of the electrons in the radiation sensitive (photoresist) layer to be exposed, and in the substrate. This negative effect is called the proximity effect.
With the generally used electron beam energy of about 25 keV, the electron range in the photoresist and in the substrate is approximately 5 .mu.m before being decelerated through collisions with atoms and electrons. This causes a lateral scattering of the impinging electrons by approximately that amount, and consequently a considerable blurring of the patterns transferred.
The effects of such scattering is shown in FIG. 1. FIG. 1A represents an insulated pattern element A in a photoresist layer 1; this pattern element can, e.g., be generated through the raster-like illumination with a fine electron beam, or through the projection of a corresponding mask opening with an electron beam of a greater diameter. The electrons impinging in area A are partly absorbed there; another part (represented by arrows), however, migrates outside area A and is thus lost for the exposure of the photoresist in area A. For compensating these scattering losses area A is therefore to be given a higher irradiation dose.
However, the higher irradiation dose to be applied depends on the immediate surroundings of the respective pattern element. If two pattern elements are closely adjacent, e. g., areas B1 and C in FIG. 1B, each element emits scattering electrons to the neighbouring element from which it also receives scattering electrons. Therefore the overall dose received in each of elements B1 and C is higher than in an isolated element B2.
For correcting the proximity effect it had therefore been suggested to subject the individual partial areas of the pattern to electron beams of differing intensity, and to determine the respective dosage in such manner that scattering electrons of other adjacent pattern elements are taken into consideration. A corresponding method is described in the article by Mihir Parikh, "Proximity Effect Corrections in Electron Beam Lithography", in J. Vac. Sci. Techn., Vol. 15, No. 3, 1978, pp. 382-391. To determine the respective irradiation dosage demands, however, extensive arithmetic operations (in a computer); furthermore, a local change of the irradiation dosage can be effected in an electron beam exposure system only where the electron beam is guided as a fine pen over the respective surface to be exposed (raster or vector scan principle).
However, electron beam exposure systems with raster-like beam deflection need long exposure times and are therefore of restricted use only for an economic semiconductor production. Electron beam projection systems where an exposure mask is imaged through an expanded electron beam following the shadow projection principle do not have this disadvantage, but such systems do not permit a local change of the exposure dosage, and consequently no compensation of the proximity effect. The practical use of electron beam projection systems for making highly integrated circuits is therefore questionable. For that reason it has been suggested to disregard an electron beam projection exposure, and to use instead ion beams where owing to the higher ion mass scattering effects play a very minor part only. However, such ion systems are more complex with regard to beam generation and control.
Another way of compensating the proximity effect which in principle would also be applicable to electron beam projection systems consists in exposing the individual pattern elements not in their nominal dimensions but in using an exposure area larger than the respective pattern element. In the development of the photoresist the proximity effect causes a shrinkage to the respective nominal dimensions.
To fix the exact dimensions of the area to be exposed considering the future shrinkage is, however, highly complicated for intricate patterns, and it can no longer be executed with automatic calculation methods. Consequently this method cannot be applied as a practical means for production; a correction of the proximity effect can under these circumstances only be performed by a variation of the irradiation dosage.
It is therefore an object of the present invention to provide a quickly realizable method for compensating the proximity effect that can be used in electron beam projection systems. Furthermore, a measuring method for the proximity effect is to be given which is easy to apply.